1. Technical Field
The invention relates generally to subframes for heavy-duty vehicles, such as tractor-trailers, and in particular to movable subframes for tractor-trailers. More particularly, the invention is directed to a movable subframe for tractor-trailers which includes a pair of main members formed with openings for receiving pins of a retractable pin mechanism, for locking the movable subframe in a selected position relative to the tractor-trailer body. Still more particularly, the invention relates to a movable subframe for tractor-trailers in which each one of the main member openings is shaped in a manner that minimizes undesirable distortion of the main members adjacent the upper portion of the openings.
2. Background Art
Movable subframes, typically referred to as slider boxes, slider subframes, slider undercarriages, or slider secondary frames, have been utilized on heavy-duty vehicles such as tractor-trailers or semi-trailers for many years. One or more axle/suspension systems usually are suspended from a single slider box. For purposes of clarity, hereinafter the present invention will be referred to as a slider box. It is understood that a slider box outfitted with usually two axle/suspension systems typically is referred to as a slider or slider tandem, and again, for purposes of clarity will hereinafter be referred to as a slider tandem. Of course, as mentioned above, a slider box may also be outfitted with a single axle/suspension system, or three or more axle/suspension systems. Reference herein shall be made to a slider tandem by way of example, with the understanding that such reference applies to a slider outfitted with one, two, three or more axle/suspension systems. The slider tandem in turn is mounted on the underside of the trailer frame, and is movable longitudinally therealong to provide a means for variable load distribution and vehicular maneuverability.
More specifically, the amount of cargo that a trailer may carry is governed by local, state and/or national road and bridge laws, and is dependent on proper load distribution. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as limit the maximum load that can be supported by individual axles. A trailer having a slider tandem gains an advantage with respect to laws governing maximum axle loads. More particularly, proper placement of the slider tandem varies individual axle loads or redistributes the trailer load so that it is within legal limits.
Once properly positioned, the slider tandem is locked in place on the underside of the trailer by a retractable pin mechanism. The retractable pin mechanism generally includes two or more, and typically four, retractable pins which may be interconnected by a crank mechanism. When the pins are in their extended position, they each extend through a respective opening formed in the main members of the slider box and a selected aligned one of a plurality of openings formed in rails of the trailer body. The pins thereby lock the slider box in a selected position relative to the trailer body.
The prior art openings formed in the main members of the slider box for receiving the retractable pins typically are round, or circular, which leads to certain disadvantages. More particularly, the openings are formed in a flat sheet or plate of metal such as steel, which is then bent into a usually C-shaped cross section to form one of the main members of the slider box. The openings are located near the upper bend of the C-shaped cross section, which causes undesirable distortion of the slider box main member at the top of the openings, because the portion of the main member at the top of each opening does not form properly with the curvature of the bend radius. This distortion interferes with the selective movement of the slider box main members relative to the trailer frame rails. When lower-strength materials are used for the slider box main members, the distortion is minimal because these materials generally allow a small bend radius at the upper bend. However, when higher strength materials are used as is typical, a larger bend radius is required to form the C-shaped cross section of the slider box main member, leading to greater distortion of the member at the top of the openings. This increased distortion leads to a significant interference of the selective movement of the slider box main members relative to the trailer frame rails, which requires a post-forming operation to alleviate, such as grinding of the main members at the distortion. The grinding operation increases the time and effort required to produce a slider box that is capable of being repositioned relatively easily.
As a result, a need has existed in the art to develop a slider box including main members with pin openings that reduce or eliminate the distortion that occurs in the main members at the top of each respective opening, particularly when higher strength materials are used. The present invention satisfies this need, providing a slider box including main members with openings that are shaped to minimize such undesirable distortion.